Recovery of hcn tetramer



United States RECOVERY OF HCN TETRAMER Don E. Carter, Dayton, Ohio,assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation ofDelaware No Drawing. Application October 28, 1955 Serial No. 543,601

12 Claims. (Cl. 260-4655) This material can be calleddiaminomaleinitrile, although there is evidence that if often reacts asthe imino-modification. The tetramer can be reacted with glyoxal to form2,3-dicyanopyrazine.

It has been pointed out in the literature that the polymerization ofhydrogen cyanide can take various courses. The particular coursefollowed is greatly dependent upon the reaction conditions andenvironment. The classical catalyst employed for preparing the tetrameris potassium cyanide (KCN). An important objection is the low proportionof tetramer in the total polymer formed. When hydrogen cyanide ispolymerized in the presence of water, little or no tetramer is formed.The polymerization of anhydrous liquid hydrogen cyanide can also becatalyzed by a number of other catalysts. As disclosed in my copendingapplication, Serial No. 388,670, filed October 27, 1953, now U. S.Patent No. 2,722,540, of which the present application is acontinuation-in-part, improvements in the proportion of tetramer presentin the total polymer can be obtained through the use of a quaternaryammonium hydroxide as catalyst. However, even then a considerableproportion of polymeric material other than the tetramer is found inadmixture with the tetramer.

An object of the present invention is to provide a method for theseparation of HCN tetramer from HCN polymer containing same. Anotherobject is to provide a process of selective extraction for accomplishingthe said separation. Yet another object is to provide a class of liquidorganic solvents in which HCN tetramer is preferentially soluble in thepresence of the other components of total HCN polymer. Yet anotherobject is to provide such solvents which are not only highly selectivefor HCN tetramer but also have a high solvent capacity for HCN tetramer.Yet another object of the invention is to provide selective solvents forHCN tetramer which do not react with the tetramer during extraction andrecovery procedures. Other objects and advantages of the invention willbe apparent to one skilled in the art from the accompanying disclosureand discussion.

The essence of the present invention lies in the use of liquid aliphaticnitriles as selective solvents to recover HCN tetramer from HCN polymercontaining said tetramer. The nitriles have a solvent capacity for HCNtetramer much higher than most other organic solvents.

atent O are highly selective for the tetramer in addition to having ahigh capacity for the tetramer, thus provides a highly eflicient andeconomical procedure for separating tetramer from total polymer.

These factors are illustrated in the following specific examples,wherein the selectivity, and the solvent capacity,

of acetonitrile, propionitrile and butyronitrile, are shown incomparison with diethyl ether, methanol, and ethanol, all the data beingobtained under comparable conditions. It will be appreciated, of course,that variations in the proportion of polymer to solvent, thetemperature, and other details of these examples can be made withoutdeparting from the invention.

Examples [-6 Liquid anhydrous hydrogen cyanide was polymerized bycharging 525 g. anhydrous hydrocyanic acid, i. e., hydrogen cyanide, and5.1 g. tri-n-butylamine catalyst to a stainless steel tube of 2 inchdiameter and 20 inch length which was closed at one end. Low boilingmaterials had been removed from the HCN by fractional distillation andthe HCN had been dried by distillation through calcium sulfate.

The tube was placed upright in a water bath thermostated at l8i2 C.,stoppered, and vented through a tube containing calcium sulfate. Aftersix days the liquid remaining in the tube was decanted and the polymerin the tube collected. The HCN remaining in the polymer was allowed toevaporate. The weight of dry polymer remaining was 103 g. Its tetramercontent was approximately 15 weight percent.

Single-stage extractions of total HCN polymer prepared as justdescribed. were effected with severalsolvents. Five grams of the HCNpolymer and 20 grams of each of the solvents listed below were weighedinto clean tubes. The tubes were stoppered, shaken and placed in a waterbath maintained l8- *;2 C. After hours, during which time the tubes wereshaken occasionally, 5 ml. aliquots of the supernatant liquid, i. e., ofthe extract phase, were withdrawn and evaporated into a stream of drynitrogen to remove the solvent completely, leaving only polymer residuewhich consisted of the polymer which was dissolved in the liquidsolvent. The residues were weighed,-

and their HCN tetramer contents determined by spectrophotometry.

HCN Tet- Tetrarner (g.) Example Residue ramer In 5 ml.

No. Solvent (g.) (Wt. Aliquot of One Percent) Equilibrium Stage Methanoll 0.2631 27. 1 0.0712 Ethanol O. 1113 24. 8 0.0277 Diethyl ethe 0.012033. 3 0. 0040 Acetronitrile 0. 1888 35. 7 0.0674 Pronionitrile 0. 173041. 2 0. 0713 n-Bntyronitrile 0. 1444 57. 5 O. 0830 1 Methanol acteddifferently from the other solvents, in that it left a residue muchdarker than that obtained from the other solvents, which residue wasvery difficult to purify by recrystallization. Thus, it appeared toextract something that the nitriles and diethyl ether did not, extract.The same was true to a less noticeable extent with ethanol.

Of the solvents, methanol, ethanol, and diethyl ether, ethanol had thelowest selectivity for tetramer and only moderate solvent capacity.Methanol had the largest procedures for recovery of the tetramer. Theselectivity" of diethyl ether for tetramer was somewhat lower than thatof the acetonitrile, and thequantity of polymer dis solved in diethylether was quite small. Onwtheother hand, the solvent capacity ofacetonitrile for polymer was more than 15 times greater than that ofdiethyl ether.

Despite the good solvent capacity of acetonitrile for polymer, it wasfound to have good selectivity for the tetramer over other components ofthe polymer, better than methanol, ethanol, or diethyl ether. When onenow considers propionitrile, its solvent capacity is somewhat less thanthat of acetonitrile, while its selectivity is higher. Butyronitrile haseven less solvent capacity for the polymer,although still more than 10times greater than that:

of diethyl ether, and its selectivity is extremely high, almost twicethat of the selectivity of diethyl ether. Ac-

cordingly, butyronitrile is preferred for maximum efii- The last columnof the table is obtained by multiplying the grams residue times theweight percentciency.

tetramer therein, thus giving the total weight of tetramer extracted by5 ml. solvent under the experimental conditions, which are believed tohave provided essentially equilibrium between polymer and solventphases. It is seen thatbothpropionitrile and butyronitrile are moreefficient than acetonitrile, butyronitrile being most outstanding. Allof thesenitriles are.tremendously more effective than is the diethylether. Additionally, one is not faced with the explosion hazards ofdiethyl. ether. It

should also be noted that the nitriles do not react with.

the tetramer during extraction, whereas certain other solvents mentionedin the prior art, particularly acetone, appear to .so react. Althoughthe efilciency (weight tetramer per unit amount of solvent) of methanolwas the same as propionitrile, it is. unsuited to selective extractionof tetramer because it also extracts an'undesirable very dark fractionof the polymer.

Preferred solventsfor use in the present invention are the. saturatedlower aliphatic nitriles, particularly those containing from 2 to 8carbon atoms per molecule, the preferred groupyamongst these beingthosecontaining from .2 to 4 carbon atoms per molecule; It'may be noted thatthe nitriles can also be described by use of the term cyanide, i. e.,acetonitrile is methyl cyanide, propionitrile is ethyl cyanide, etc.Thus, the preferred solvents can also be called the lower alkylcyanides. Included are all of the various configurations ofthe carbonatoms in straight chains and branched chains. Thus, the S-carbon atomnitriles'include n-butyl cyanide, isobutyl cyanide;'sec.-butyl cyanide,and tert.--butyl cyanide. While the saturated nitriles are preferred notonly because of their high selectivity and solvent capacity but alsobecause of availability and inertness thusfacilitating recovery andre-use, it is not outside the 'broad'scope of the invention' to employunsaturated nitriles, e. g., acrylonitrile, meth acrylonitrileor'dinitriles, or nitriles containing non-hydrocarbon groups that areessentially non-reactive with the components of: the HCN polymer, e. g.,succinonitrile, B-chloropropionitrile. Itshould be understood that theword liquid" in the term liquid aliphatic nitrile merely means that thenitrile is liquid under the conditions of the extraction operation.Thus, a normally solid nitrile, i. e., one solid at ordinary roomtemperature (20 C.), can be used at higher temperatures at which it is aliquid. It should also be understood that mixtures of two or morenitriles-can 'be employed-asmay be desired. Generally speaking,1however,the: use of a single nitrile simplifies solvent recovery steps.

The,datargiycninthe.preceding specific examples are of course based on asingle stage extractiomcarried out under conditions such thatapproximate equilibrium-be tween solid andliqiud phases'was attained.Thoseskilled which'may be presented to a particular operator. For" someuses, a single equilibrium stage might give an extract materialsuificiently enriched in tetramer. However, it will generally bepreferred to obtain the tetramer in either pure form or at least a morehighly concentrated form than is possible in one equilibrium stage. Insuch event, resort will be had to the well-known techniques, for examplecountercurrent decantation or various of the other extraction procedureswherein extraction is eifected in a plurality of stages. Thus, a singlebatch of polymer may, be successively extracted with increments of freshsolvent, 7 until tetramer has been removed from the polymer to as greatan extent as is desired. In such operations, the first extract phasewill contain extract material richest in tetramenwhile the last extractphase will be highly contaminated with non-tetramer polymeric'material.Any or all of the extract phases can be subjected to evaporation toremove solvent from the polymer content, or can be chilled toprecipitate out polymer, or can have a nonsolvent, e. g., hexane orbenzene, added thereto to precipitate out ploymer. Polymer extracts thusobtained can be subjected to further extraction for further purificationto as great an extent as is desired. The most efficient operation, sofar as extracting tetramer from total polymer, is had by contactingpolymer and solvent countercurrently in a plurality of stages, either asseparate batch contactings or in any of the well-known apparatus forcontinuously countercurrently contacting solid with solvent. The solventcan be introduced into one end of a series of extraction and separationstages alternated with each other and passed continuously therethroughto the other end of the series, polymer meanwhile being introduced atsaid other end orat an intermediate point in the series and passed inthe direction generally opposite to that of the solvent. Each contactingstage is followed by a separation-stagefrom which solvent is then passedto a contacting stage farther along the'linein the direction of fiow'ofsolvent while separated polymer is passed in the opposite direction inthe lineato the contacting stage from which the solvent had come.Polymer can be mechanically moved against a stream of flowing solvent,as by placing the polymer in perforated baskets-or in a pluralityofseparate compart ments having on each end a filter medium such asfilter cloth and moved mechanically through 'a conduit opposite" to thedirection of flow of solvent. In any countercurrent scheme ofextraction, it is possible with an extent of contact'amounting to asufficient number of theoreticalequilibrium stages to recoverundissolved polymer residue essentially completely stripped of tetramer.Solvent removed from the stage into which fresh total polymer is'introduced will contain dissolved polymer which is richest in tetramer.Further enrichment can be obtained by passing such solventcountercurrent to further polymer" beyond the point of entry of totalpolymer feed to the extraction process, this further polymer (other thanthe feed polymer) being supplied by introducing into the sol-' ventjustprior to its leaving the system, essentially pure tetramer andcausing it to flow countercurrent to the sol-' vent whereby a backwashor refluxing action is obtained. In this type of system, it ispossibleto recovera final extract phase containing essentially puretetramer or'extract apparent to those skilled in the art, can beemployed in practicing the invention.

The extraction can be effected over a comparatively wide range oftemperatures, such as those ranging from just above the melting point ofthe nitrile solvent up to the boiling point of the nitrile solvent.Preferably the temperature will not exceed 150 C., since decompositionof tetramer is apt to set in at about 180 C. and higher. Generally, itmay be stated that temperatures within the range of to 80 C. are quitesatisfactory, and ordinary room temperature will often be used, since itpermits efficient extraction and avoids the expense of refrigeration orheating. While the extraction can be effected at pressures below orabove atmospheric, there would seldom be suflicient advantage to warrantit.

While the invention has been described with particular reference tovarious preferred embodiments thereof, it will be appreciated thatnumerous modifications and variations are possible without departingfrom the invention.

I claim:

1. A process which comprises selectively extracting HCN tetramer fromHCN polymer containing same, employing as solvent a liquid alkylnitrile.

2. A process according to claim 1 wherein said nitrile is an alkylcyanide containing four carbon atoms.

3. A process according to claim 1 wherein said nitrile isn-butyronitrile.

4. A process according to claim 1 is propionitrile.

5. A process according to claim 1 wherein said nitrile is acetonitrile.

wherein said nitrile 6. A process according to claim 1 wherein saidextracting is effected in a plurality of stages.

7. A process according to claim 6 wherein polymer and solvent arecontacted countercurrently.

8. Method of separating HCN tetramer from HCN polymer containing saidtetremer, which comprises intimately contacting said HCN polymer with aliquid alkyl nitrile, separating an undissolved polymer residue poor intetramer from a liquid extract phase comprising said nitrile anddissolved polymer rich in tetramer, and recovering tetramer-rich productfrom said extract phase.

9. Method according to claim 8 wherein said nitrile is an alkyl cyanidecontaining four carbon atoms.

10. Method according to claim 8 wherein said nitrile is n-butyronitrile.

11. Method according to claim 8 wherein said nitrile is propionitrile.

12. Method according to claim 8 wherein said nitrile is acetonitrile.

References Cited in the file of this patent UNITED STATES PATENTSWoodward Mar. 7, 1950 Carter Nov. 1, 1955 OTHER REFERENCES

1. A PROCESS WHICH COMPRISES SELECTIVELY EXTRACTING HCN TETRAMER FROMHCN POLYMER CONTAINING SAME, EMPLOYING AS SOLVENT A LIQUID ALKYLNITRILE.